Direct-Bonded Copper (DBC) substrates (also called Direct Copper Bonded (DCB) substrates) are used extensively in the power semiconductor industry. A DBC substrate generally includes a thick ceramic substrate member, to which a thinner top plate of copper and a thinner bottom plate of copper are bonded. A semiconductor die includes one or more power devices such as, for example, power transistors and/or power diodes. Metallization on the back side of this semiconductor die is soldered to the top copper plate of the DBC. The DBC is often physically secured by soldering or welding the top copper plate to a package lead. Bond pads on the front side surface of the die are typically wirebonded to other package leads. The resulting assembly is then encapsulated. After lead trimming, ends of the leads protrude from the encapsulant. To improve conduction of heat from the package, the encapsulant is made not to cover the bottom surface of the bottom copper plate. This exposed surface of the DBC facilitates heat escaping from the package. The exposed surface may, for example, be placed in good thermal contact with an external heat sink. For additional background information on DBC substrates and packaged power devices that include DBC substrates, see U.S. Pat. No. 6,404,065.
Although DBC-based power devices work well in many applications, other applications involve thermal cycling over wider temperature ranges and to higher temperatures. In such applications, differential thermal expansion between the DBC ceramic and copper may result in cracking of the ceramic or in separation at the ceramic-to-copper interface. Accordingly, a metal bonded ceramic substrate referred to as a DBA (Direct-Bonded Aluminum) substrate has been developed. DBA substrates are sometimes called Direct Aluminum Bonded (DAB) substrates. Rather than direct bonding copper plates onto ceramic as in the case of a DBC substrate as described above, aluminum plates are direct bonded onto ceramic. Aluminum is a softer and more ductile metal than copper. Aluminum has a lower crystal hardness modulus than copper. As a result of being softer, the aluminum conforms with less stress to the expansion and contraction of the alumina ceramic substrate. Thus DBA substrates generally provide better resistance to cracking and lift off failures under severe thermal power cycling.
Aluminum is not, however, a readily solderable metal. The top surface of the top side aluminum plate of the DBA is therefore provided with a thin layer of a solderable metal (for example, copper, or nickel, or silver, or gold). This thin layer of solderable metal allows a metalized back side of a semiconductor die to be soldered to the upper surface of the top aluminum plate of the DBA. As in the case of DBC-based packages, bond pads on the front side of the die are wirebonded to package leads. Encapsulation and lead trimming are then carried out. For additional background information on DBA substrates and on packaged power devices that include DBA substrates, see: U.S. Pat. No. 6,798,060 and U.S. Pat. No. 7,005,734.
In even more severe temperature cycling applications, DBA-based power semiconductor devices have been noticed to fail. Further improvements in direct metal bonded semiconductor packages are sought.